1. Field of the Invention
The present invention is directed to a high-voltage generator for driving a shock wave source of the type which generates a shock wave in an acoustic transmission medium, as well as a method for generating a high-voltage, high current pulse.
2. Description of the Prior Art
Acoustic shock waves are used for a large variety of purposes, for example in materials research and in medical technology. In medical technology, shock waves are used for non-invasive treatment of stone maladies. The shock waves are focused on a calculus, for example a kidney stone, situated in the body of a patient and are coupled into the body of the patient and act upon the calculus to disintegrate the calculus into fragments of a size which can be eliminated (excreted) in a natural manner, or which can be dissolved with chemotherapeutic measures. It has also been suggested to treat malignant tissue, for example tumors, with shock waves.
Various types of shock wave sources are used for generating the shock waves, for example shock wave sources having an underwater spark gap, as described in German OS 26 50 624. It is also known to generate shock waves based on the electro-dynamic principle, as described in German OS 33 28 051, corresponding to U.S. Pat. No. 4,674,505. Shock waves can also be generated based on the piezoelectric principle, as described in German OS 33 19 871. All of these shock wave sources have in common the necessity of being supplied with a high-voltage pulse with high current in order to generate a shock wave. This type of pulse is usually generated with a high-voltage generator which contains a high-voltage capacitor chargeable to high-voltage, and a high-voltage switch, for example a triggerable spark gap switch. The high-voltage switch serves the purpose of connecting the charged high-voltage capacitor to the shock wave source, so that the electrical energy stored in the high-voltage capacitor suddenly discharges into the shock wave source, thereby generating a shock wave (see, for example, U.S. Pat. No. 4,674,505).
A disadvantage of these known shock wave sources is that the necessary high-voltage supply is expensive, and relatively susceptible to disruption. Additionally, the high-voltage switch can wear relatively quickly, and must then be replaced. Moreover, the wave shape (chronological amplitude curve and pulse duration) of the shock waves generated with the assistance of known high-voltage generators is difficult to adapt to the requirement of individual therapeutic cases. The capacitive, inductive and ohmic resistor components of the shock wave source form a network in common with the components of the high-voltage generator in which high-energy, pulse-like voltages and/or currents appear upon discharge of the high-voltage capacitor. Together with the acoustic properties of the shock wave source and the transmission medium (water or body tissue), the chronological curve of these voltages and currents determines the wave shape of the generated shock wave. Influencing the shape of the generated shock wave is thus only possible by modifying the electrical network formed by the high-voltage generator and the shock wave source, or by modifying the acoustic properties of the shock wave source. Both of these modifications are extremely complicated, and cannot be implemented in clinical practice. The wave shape of the generated shock wave therefore usually represents a compromise which cannot satisfy all possible therapeutic cases, namely those which have become routine, those which are under investigation in clinical research, and those which will arise in the future. Because the high-voltage supply provided for charging the high-voltage capacitor can only supply a relatively low charging current, the time required in the none high-voltage generators for charging the high-voltage capacitor is relatively long, and the maximum repetition rate of generating shock waves is correspondingly low.
The use of semiconductor components for forming the high-voltage switch is not possible, because semiconductor components cannot withstand the necessary high-voltages and high currents which occur during operation.
It is also known to drive the shock wave source with a generator constructed similar to an ultrasound transmitter. The shock wave source is chargeable with pulses having different chronological curves to adapt the wave shape of the shock wave to respective therapeutic cases. Such a system is described in German OS 31 19 295, corresponding to U.S. Pat. No. 4,526,168. This type of system, however, is only suitable for comparatively low-voltages and currents, which at most suffice for the drive of certain piezoelectric shock wave sources.